1. Field of Invention
The disclosure herein relates in general to rolling cone earth boring bits, and in particular to a compact row arrangement on a rolling cone.
2. Description of Prior Art
Drilling systems having earth boring drill bits are used in the oil and gas industry for creating wells drilled into hydrocarbon bearing substrata. Drilling systems typically comprise a drilling rig (not shown) used in conjunction with a rotating drill string wherein the drill bit is disposed on the terminal end of the drill string and used for boring through the subterranean formation.
Drill bits typically are chosen from one of two types, either drag bits or roller cone bits. Rotating the bit body with the cutting elements on the outer surface of the roller cone body crushes the rock and the cuttings may be washed away with drilling fluid. A rolling cone earth boring bit has a bit body with typically three legs. A bearing pin depends from each leg and a cone mounts rotatably to each bearing pin. The cones have rows of cutting teeth on the outer surface of the cone. In one type, the cutting elements comprise teeth machined into the surface of the cone. In another type, the cutting elements comprise carbide compacts or inserts that are pressed-fitted into mating holes in the cone surface.
Compacts generally have a cylindrical base that is inserted into a hole and a protruding cutting tip. The cutting tips may have chisel, hemispherical, ovoid or other shapes. Particularly on the heel row, which is located near the gage surface of each cone, the compacts may have asymmetrical shoulder surfaces for engaging the sidewall of the bore hole. Depending upon the formation being drilled, different shapes are utilized for aggressiveness of cutting and durability.
One example of a roller cone bit 11 is provided in side view in FIG. 1, the bit 11 having a body 13 with legs 15, roller cone bits typically comprise three legs 15. A cone 17 rotatably mounts to a bearing pin (not shown) on each leg 15. Each cone 17 has a plurality of inserts 19, arranged in at least one inner row. A plurality of outer or heel row compacts 21 are adjacent to a gage surface 23 of each cone 17. In the embodiment shown, heel row compacts 21 are generally ovoid, although different shapes could be used.
An example of a roller cone 17 is provided in side view in FIG. 2. The roller cone 17 is shown as a conical structure generally symmetric about an axis AX, and having a base 18 on a lower end and an upper 43 on the opposite end. The cone 17 radius linearly decreases from the base 18 to the upper 43. The inserts 19 or compacts are disposed on portions of the cone 17 outer surface referred to as lands. The lands include an outer land 27, a first inner land 33, and a second inner land 37. The outer land 27 is defined by the cone 17 outer surface proximate to the roller cone base 18. The outer land 27 is bounded on a lower side by the roller cone base 18 and its upper side by a curved surface 28. The curved surface 28 comprises a radial indentation or groove circumscribing the cone 17 outer surface. The second inner land 37 is defined by the cone 17 outer surface proximate to the upper 43. A second curved surface 34 is disposed between the second inner land 37 and the first inner land 33.
Still referring to FIG. 2, the outer land 27 includes inserts 19 thereon arranged in a first row 29 and a second row 31. The rows 29, 31 extend along a generally circular path about the axis AX. Being disposed above the first row 29, the second row 31 follows a circular path having a diameter less than or equal to the first row. The first and second inner lands 33, 37 have a single row 35, 39 of compacts 19 respectively formed thereon.
FIG. 3 schematically represents a borehole profile of a prior art roller cone bit. The profile depicts multiple compact outlines along an azimuthal line LBH radially extending outward from a bottomhole centerline. Each compact outline corresponds to an individual compact 19, or row 35, 39 of compacts 19, on a roller cone bit. The compact outlines represent where the compacts 19 strike the bottomhole at the azimuthal line LBH during a 360° drill bit rotation. Notations are associated with each compact outline identifying which of three cones the compact 19 is located and the total number of compacts in the row where the compact is attached. For example, the compact outline denoted by 14-1 represents a compact from a row having 14 compacts from cone 1 of a drill bit.